The past twenty-five or so years have seen the development of ever smaller electrical circuit components at the chip level. However, to take fullest advantage of achievements in electrical circuit miniaturization, one must package the resultant printed circuit cards containing these chips in an efficient manner. Clearly, the packaging of printed circuit cards in tight spaces is a direct logical extension of increasing chip level circuit densities. It should also be noted that the tight packaging of integrated circuit chips on printed circuit cards and the correspondingly dense packaging of the printed circuit cards is a design goal that is carried out for more than just the convenience of compactness. Compactness provides shorter distances between circuit components which, in turn, serves the very desirable goal of being able to operate the circuits effectively at higher frequencies, thus increasing the speed of numerous different forms of electrical systems, including but not limited to data processing systems.
Moreover, mainly for reasons associated with long-term system operation and reliability, it is likewise very desirable to be able to easily insert and remove these printed circuit cards even when they are disposed in very tight spaces. The insertion and removal operations are also provided as an important part of a “hot-pluggability” function which is very desirable for “on the fly” repairs, replacements, maintenance and upgrades. Accordingly, to whatever extent possible, packaging designs should be: economical to produce; function smoothly; require little or no maintenance; be producible from inexpensive, readily available materials; and be reliably operable over a large number of insertion and removal operation cycles.
Yet one other concern arises in electrical systems as circuit feature size shrinks, operating frequencies increase and packaging densities grow larger, namely, the generation of electromagnetic interference (EMI). Electronic circuit packaging designs should thus also be compatible with structures and configurations that are employed to prevent the leakage of electromagnetic interference. To whatever extent possible, packaging designs should also include structures which actually contribute positively to the containment of electromagnetic interference. There is an ever increasing problem of electromagnetic interference caused by such devices. Virtually every electronic device, intentionally or not, emits some form of electromagnetic radiation. While this condition could be tolerated when few devices existed, the increasing number of electronic devices has made the problem more acute. The problem has been exacerbated by the “improvement” in semiconductor devices which allows them to operate at higher speeds, generally causing emission in the higher frequency bands where interference is more likely to occur. This is especially true with the incorporation of optical modules operating at very high speeds. Successful minimization of the interference problem, sometimes referred to as “electro-magnetic compatibility” or “EMC”, generally requires that emissions from a given device be reduced by shielding and other means, and that shielding be employed to reduce the sensitivity of a device to fields from other devices. Since shielding helps to reduce sensitivity to external fields as well as reduce emissions from the device, it is a common approach to a solution of the problem.
In newer high speed packages it is necessary to use a metallic type of gasket to provide better conduction with an electrical enclosure in which the printed circuit cards are engaged. For example, optical riser card assemblies include a plurality optical modules mounted on a single printed circuit card that require an EMC gasket between the housing of the optical module and the tail stock of the electrical enclosure (e.g., a docking cassette). The tail stock of the docking cassette includes at least one opening corresponding to a cable opening of each optical module. Each optical module is commonly a receiver and/or a transmitter configured with a cable opening to receive a cable connector of a corresponding I/O cable. However, one vendor may not be able to supply all of the optical modules needed and optical modules having different mechanical packaging from other vendors may be supplied to make up for this deficit. In this case, the EMC gasket may not be compatible with differently sized optical modules from these other vendors.
It is also noted that the present discussion refers to printed circuit boards and printed circuit cards. As contemplated herein, the printed circuit board is the larger component into which at least one printed circuit card is inserted for purposes of electrical connection. The present disclosure places no specific limits on either the size of a printed circuit board or the size of a printed circuit card. In the most general situation, a circuit board will be populated with a plurality of printed circuit cards. That is, the printed board will have a number of printed circuit cards inserted therein. Accordingly, as used herein, the terms “printed circuit board” and “printed circuit card” are considered to be relative terms.
Accordingly, a need exists for a method and apparatus for a universal EMC gasket that is transparent to the size of the electrical or optical module packaging and provides EMC shielding for a variety of differently sized electrical or optical modules from different vendors. The universal EMC gasket must be mechanically stable to ensure a continuous grounding and must be designed to facilitate assembly and teardown. In addition, it is desired that the assembly and manufacturing costs for a method and apparatus for shielding electrical and optical modules having a variety of mechanical packages be reduced.